1. Field of the Invention
The present invention relates generally to a method and system for diagnosing a fuel cell stack and, more particularly, to a method and system for more accurately diagnosing a state of a fuel cell stack based on control logic between control devices.
2. Description of the Related Art
A fuel cell vehicle is a type of vehicle that includes a fuel cell stack that laminates multiple fuel cells used as a power source; a fuel supply system that supplies hydrogen which is a fuel for the fuel cell stack; an air supply system that supplies oxygen which is an oxidant necessary for electrochemical reactions; and a water and heat management system that adjusts a temperature of the fuel cell stack. The fuel supply system decompresses compressed hydrogen within a hydrogen tank and supplies the hydrogen to a fuel electrode (anode) of the stack. In addition, the air supply system supplies air suctioned from exterior using an air blower to an air electrode (cathode) of the stack.
When hydrogen and air are provided to the fuel electrode and air electrode of the stack, respectively, hydrogen ions are extracted through a catalysis reaction at the fuel electrode. The separated hydrogen ions are delivered to the anode through an electrolyte membrane, and the hydrogen ions and electrons from the fuel electrode produce an electrochemical reaction with oxygen to generate electric energy at the anode. Specifically, an electrochemical oxidation of hydrogen at the fuel electrode and electrochemical reduction of oxygen at the air electrode induces the electrons to move, and moving of the electrons generates electricity and heat. Additionally, water vapor or water is generated by the chemical action of combining hydrogen with oxygen.
An emission device is arranged to emit unreacted hydrogen and oxygen, and by-products produced in the procedure of the electric energy generation such as water vapor, water, and heat. Further, gases such as water vapor, hydrogen, and oxygen are emitted to the air through a ventilation hood. Components for operating a fuel cell, such as an air blower, a hydrogen recirculation blower, a water pump, and the like, are connected to a main bus terminal to facilitate an operation of the fuel cell. The main bus terminal may be connected to various relays for facilitating power interruption and connection, and a diode for preventing a back current.
Dry air supplied through the air blower is humidified by a humidifier, and is then supplied to a cathode of the fuel cell stack. The exhaust gas of the cathode, which has substantially high humidity due to the water generated therein, is transferred to the humidifier, and may be used to humidify dry air to be supplied to the cathode by the air blower. Further, a state and performance of a fuel cell stack is determined by high-precision response to operating conditions such as an air temperature, coolant temperature, and current. Continuous driving under poor operating conditions causes performance degradation of the fuel cell, and leads to potential failure to produce adequate output power demanded by a driver in the short term. Also, in the long term, this may cause durability deterioration and reduction of a life cycle of the fuel cell.
Moreover, dry-out of the stack is attributable to two main causes including a dry-out that occurs at a substantially high temperature and high output, and another dry-out that occurs at a substantially low output. The dry-out at a high temperature and high output occurs from losing a heat balance within the stack; while the low output dry-out occurs due to air oversupply. When dry-out of the fuel cell stack occurs, output of the fuel cell stack decreases, and the time required to recover normal output increases. Accordingly, it is necessary to detect whether dry-out of the fuel cell stack occurs, and to adjust conditions to rapidly recover from the dry-out via stack recovery driving.
For diagnosing a state of the fuel cell stack, a dry-out state caused by long-term deterioration of the fuel cell stack and shortage of water content within a membrane electrode assembly may be determined using an ohmic value. To measure the ohmic value, an interrupter disposed between the fuel cell stack and external energy consumption device may be used. The ohmic value may be measured based on a sudden increase in voltage that results from temporarily interrupting a current flow in the fuel cell stack. However, using the interrupter may create a heat problem within the external energy consumption device, and may contribute to the lack of durability of the interrupter or external energy consumption device.